Conventionally known marking methods make use of a mechanical device in order to transfer e.g. ink contained in an ink pad to the surface of an object in order to provide it with a stamp carrying information thereupon. An individual stamp in an automated process is repeated for subsequent objects. Because of its mechanical nature and the drying time associated with ink, an ink stamping process is relatively slow. Moreover, if the mark is accidentally touched prior to complete drying, the mark will smudge. Another problem associated with ink stamping methods is that the quality of ink stamped marks may substantially vary with time. This variation may be dependent upon the quantity of ink applied, ambient temperature and humidity, and/or the condition of the surface of the stamp. In any event, the consistency of a stamped mark may vary widely from one object out of a series to another.
Moreover application of ink is not always preferable, more particularly, when the material has to pass e.g. through wet processing solutions.
As a result of the deficiencies associated with ink stamping, it has become increasingly popular to use a laser beam to mark the surface of an object. Unlike ink stamping, laser marking is very fast, requires no curing time, has a consistently high quality, and can take place at the end of the manufacturing process. Moreover laser markings more easily withstand passage through processing solutions than ink stamps.
Laser beam marking thus provides an economic and rapid method of writing, bar coding and decorative marking of plastics in general and is an advantageously used technique over current printing technologies because of the ease at which the layout can be adjusted using graphic computer programs and ease of integration into a production line. In some applications laser marking is a well known and important means for quickly and cleanly inscribing by irradiating a target surface, like e.g. a plastic surface, with laser light, thereby providing it with permanent informational indicia marks, such as characters, letters, figures, symbols, bar codes or images, date codes, batch codes, bar codes or part numbers, functional marks, such as computer keyboard and electronic keypad characters, and decorative marks, such as company logos. In some applications such laser marks are moreover made visible and readable in a dark or dimly lit environment as e.g. in order to read informational indicia on items, such as clocks, emergency exit signs, safety information signboards, interior automobile control buttons, and the like. The term “indicia” further refers to any laser mark including, but not limited to, alphabetical characters, numbers, drawings or patterns. Laser marking is a contact-free procedure: it makes marking possible, even on soft, irregular surfaces that are not readily accessible. Laser marking is ink-free, which provides long-lasting applications and it is solvent-free, which makes it more ecologically acceptable and resistant to passage in processing baths.
Making use of a laser in order to mark an object suitable for this application is a fast and economical means of marking. However certain disadvantages associated with state-of-the-art laser marking techniques that merely burn the surface in order to achieve the desired mark, as e.g. an image on the surface of a chip clearly visible at nearly every angle of incidence to a light source.
A mark burned in a surface by a laser, on the other hand, may further contain contaminants deposited on the surface subsequent to marking and those contaminants may blur or even obscure the mark. Additionally, because the laser actually burns the surface of the work piece the associated burning may cause damaging because of temperatures exceeding acceptable limits.
Surfaces not suitable for being marked by a laser may moreover require laser reactive coatings to be applied onto their surface, wherein application of such coatings may take additional manufacturing procedures and manufacturing time. Nevertheless as in U.S. Pat. No. 4,769,310 a method for laser marking of ceramic materials making use of an inorganic pigment has e.g. been described as an illustration that not only soft substrates can be laser marked.
With respect to conventional edge marking in the particular application field of photographic film manufacturing it is well-known that printing involves some sort of identification indicia along the edge of film rolls during the finishing operation. Edge marked films provide direct verification of roll identity, sheet identity and waste identity during all stages of the manufacturing process. Edge marked films moreover provide accurate footage identification that enables operators to quickly identify, trace and remove film imperfections, thereby minimizing the amount of product waste. The importance of edge marking of film becomes clear from the fact that it increases process understanding by allowing process interactions to be more closely identified with their corresponding effect on the product.
Furtheron traditional embossing marking techniques may be replaced by laser edge marking as current mechanical embossing techniques by embossing wheels are not accurately programmable, generate poor quality marks and require excessive maintenance. Laser edge marking however is particularly advantageous in industry because it provides a permanent record and can be read before and after film processing. Advances in laser technology provided use of dot matrix CO2 laser marking systems in order to replace existing embossing technologies. Apart from an undeniable advantage of laser marking of film at a required throughput rate, an unacceptable level of fog spots may occur for marking light-sensitive photographic films, due to light flashes of the laser burning process. A further shortcoming resulting from application of these advanced high powered laser systems used for edge marking photosensitive film is that by-products, produced during marking may impinge on the film surface. Laser energy by-products in form of smoke and irradiated debris on the film surface are known to further cause the localized fogging on the film, even when the film is flushed in an inert gas atmosphere, substantially free from oxygen.
More recent developments in laser technology enabled the development of high speed marking systems using short pulse lasers. Short pulse laser exposure on photosensitive film show promising results in reducing the occurrences of fog spots. Air jets directed at the laser impingement point on the film surface are further known to reduce the occurrence of fog. Statistical methods have been employed to gain information on fog incidence reduction when laser marking photosensitive film. Moreover laser pulse width has hitherto been thought not to have a significant effect on fog, opposite to laser peak power.
There are several laser types available for marking plastic surfaces. CO2 lasers allow speeds up to 10,000 mm/sec. At 10,600 nm the CO2 laser enables laser marking by thermochemical reaction, melting, vaporizing and engraving. A Nd:YAG laser allows speeds up to 2000 mm/sec. The Nd:YAG laser at 1064 nm provides laser marking by carbonization, sublimation, discoloration, foaming and engraving. At lower power levels at 532 nm, the Nd:YAG laser marks by leaching or selective bleaching of dyes and pigments. These lasers have good flexibility in text and imaging and broad flexibility in marking based on several phenomena, such as melting, foaming, vaporizing and engraving. An excimer laser with the frequency in the range of 196–351 nm leads to the marking of plastic surfaces by photochemical ablation or reaction.
Conventionally the name of a maker, the kind of film, the effective usable period and the lot No. are marked on a photosensitive material such as an X-ray film, since it is necessary to confirm the marks in the undeveloped state of the photosensitive material due to the convenience on the handling.
More recently an apparatus for marking indicia on a moving photosensitive web has been described in EP-A 1 120 685 wherein said apparatus comprises a source of laser energy; a laser printer means operably connected to said source of laser energy, said laser printer means being provided with a laser head; a laser beam tube connected to said laser head, said laser beam tube having an active end; and, a nozzle element structurally associated with said active end of said laser beam tube, said nozzle element comprising a chamber having a laser energy inlet end and a laser energy outlet end; an air jet member arranged in said chamber for directing a burst of air onto a laser beam impingeable surface; at least one lens arranged in said chamber for focusing each one of a plurality of laser beams passing through said chamber; a lens cleaning member arranged in said chamber proximate to said at least one lens; and, a vacuum port extending from said chamber, said vacuum port providing means for evacuating said chamber of smoke and debris generated during laser marking; and wherein said laser head has a plurality of lasers disposed therein for generating a plurality of laser beams, a lens arranged in said laser beam tube for focusing said each one of a plurality of laser beams along a predetermined optical path through said laser beam tube and into impinging contact with said moving photosensitive web thereby producing said indicia thereon. Said nozzle element has been described in detail in EP-A 1 120 684. Working with such a nozzle element however lays burden on the velocity with which the moving web can run relative to the laser apparatus, the more as nowadays very high laser marking speeds are envisaged.
The problem thus becomes aggravated as dust stick to the film surface is reinforced, as sparks occur and as harmful fog is caused in the vicinity of the mark on the film by the sparks.